How do I model the secondary reflection coming from the inside of the back surface of glass?
Don't use Clamping (especially with such a low value of 1). It prevents the lights from being simulated realistically in something as complex as glass. Turning on Caustics also helps.

Here are some render comparison:



To me, the glass material is unnecessarily complex, unless you have a good reason to use it (for dispersion, etc), I suggest you looking into the Principled Shader. With one shader, we can achieve even better caustics and faster renders.